Defects in additive manufactured metals and their effect on fatigue performance: A state-of-the-art review
Introduction
Additive manufacturing (AM) is a method through which parts are created by additive processes as opposed to the conventional subtractive processes. These technologies were first targeting rapid prototyping to assist the design process. By the emergence of more advanced technologies, the properties of the manufactured parts improved to meet the expectations of various industrial purposes. These processes can currently utilize metals, ceramics, bioengineered tissues, and various polymers as the feedstock material.
The technology of AM, which is directly informed by 3D model data, is being developed rapidly as a potential production method in several industries. This technology offers significant advantages such as delivering intricate and complex geometries and short lead times. Powder and wire are forms of feedstock commonly used in metal AM. The powder or wire is melted by a focused heat source generated commonly by a laser beam or electron beam and subsequently cooled to form a part. As the AM technologies are being industrialized, the quality of the manufactured parts is improving, creating broader applications for these parts. Technologies like laser additive manufacturing (LAM) are being introduced exponentially to fields such as biomedical research, aircraft, and space industries. These methods can produce metal components with desirable properties for a variety of applications.
AM process parameters, the formed microstructure and defects, and consequently the mechanical properties of the manufactured parts have significant implications on their structural integrity. Operation of the components made by AM for applications such as in biomedical and aerospace industries, should be reliable under a wide variety of complex dynamic loadings in different environmental conditions. These loading conditions are oftentimes multiaxial even in parts under uniaxial loading due to complex geometry or interaction of residual stresses and presence of AM defects.
AM parts have shown a finer microstructure compared to the conventionally made parts, which leads to a relatively good static strength. Also, since defects have a less significant effect under static loading as opposed to cyclic loading, parts generally meet the standards and specifications on tensile properties for industrial use. Metal AM fatigue performance, on the other hand, is significantly affected by presence of defects. Comprehensive investigations are required to study the characteristics of AM defects and operation of AM components containing defects under dynamic loading conditions. High cost full-scale testing in environments resembling the working conditions, might be necessary to achieve the required level of reliability. However, testing could be reduced by development of applied analytical performance prediction techniques based on the intrinsic defects of AM parts.
Metal AM parts contain a variety of defects such as Lack of Fusion (LOF) defects that have also been reported in welds, un-melted particles as reported in Powder Metallurgy, and gas porosities, as detected in castings. Cracks could also rise from accumulated residual stresses in metal AM parts during the manufacturing process and the resultant distortion [1]. Since fatigue cracks usually start at stress concentrations like pores and inclusions [2], these defects have a significant effect on the fatigue life of AM components [3], [4], [5], and are the key contributors to the inferior fatigue properties of metal AM parts compared to their wrought counterparts [6], [7]. These imperfections also promote localized corrosion attacks and consequently stimulate fatigue cracking [8]. Therefore, metal AM defects should be characterized, analyzed, categorized, and ranked based on their importance and the degree that they affect fatigue performance. Non-destructive characterization methods combined with predictive models could help to predict fatigue performance based on the microstructural features and defect content.
Processing and post-processing strategies influence the fatigue performance of metal AM parts through altering microstructure and defects, and consequently the material sensitivity to the existing defects. AM processes should be optimized to improve fatigue performance of the AM parts. The main attributes to target to improve fatigue performance are residual stresses, surface roughness, internal defects, and microstructure. Variation of the final microstructure and defect content throughout the component might be seen even in one large component with complex geometry, mainly due to temperature gradient variations and heat transfer parameters at different locations during the process. Therefore, post-processing might be still necessary for metal AM parts to remove tensile residual stresses, achieve a uniform microstructure and defects content, and the desired reliability of fatigue performance. The challenge is to improve the quality of net shaped metal AM parts and to reduce the amount of required post-processing procedures.
Metal AM processes such as EBM and SLM and their comparison for various applications have been the focus of several papers such as works by Oliveira et al. [9], Liu and Shin [10], Zhang et al. [11], and Sing et al. [12] in which the different processes and their general mechanical properties has been discussed. Liu and Shin had gathered key fatigue properties for Ti-6Al-4V such as fatigue limit and fatigue long crack growth threshold based on the process (EBM, SLM, DED, wrought, cast, forged), post processing and surface finish, specimen orientation, and the load ratio. Their comparisons help to better understand the relations between processing parameters, resultant microstructures and associated mechanical properties which would be useful for modeling the fatigue performance of AM metals. They also have emphasized the critical effect of defects in as-built AM Ti-6A-l4V components on mechanical performances. They concluded that α′ martensite microstructure of DED and SLM Ti-6Al-4V are responsible for the lower crack thresholds, but higher fatigue limits as compared to EBM, wrought, forged and heat treated Ti-6Al-4V and confirmed the positive effect of surface machining and heat treatments on the fatigue performance of AM fabricated Ti-6Al-4V as will be further discussed in this work.
In recent years several review papers such as the works by Yu et al.[13], Lewandowski et al. [14], and Yap et al. [15] had addressed the state of the art in metal AM processes which contain valuable information on various AM technologies, their advantages and limitations, and the application of the AM metals based on their different properties. However, this review focuses on the effect of intrinsic metal AM defects, both surface and internal, on fatigue performance as one of the most critical properties of the AM metal components. As mentioned earlier AM metal components specifically in aerospace and biomedical applications need to perform under complex dynamic loading conditions and intrinsic metal AM defects significantly affect fatigue performance of these AM components.
A great fraction of the available literature is on metal PBF constant amplitude uniaxial fatigue performance. Also, AM Ti-6Al-4V fatigue performance is the most frequently studied. Typical metal AM processes and the sources of defect formation are mentioned, defect characterization and statistical analysis methods are explained, and the effect of these defects on fatigue performance of AM parts is discussed. Methods used throughout literature for modeling and prediction of the role of defects on the fatigue performance of AM parts are also reviewed. Finally, an outlook and perspective for future research is provided.
Section snippets
Metal AM processes and classifications
AM technologies are generally categorized, based on the state of material used, the mechanism by which layers of material are binding, and the source of energy which melts or softens the material [16], [17]. Metal AM processes categorized based on the accumulation method and energy input are shown in Fig. 1 [18]. Based on this flowchart, two main categories of technology used in metal AM are Power Bed Fusion (PBF) and Directed Energy Deposition (DED). The main power sources used in PBF are
Effect of AM defects on fatigue performance
A common representative dimension for a defect is the parameter proposed by Murakami and Endo [129], which has been used in many works to calculate the stress intensity factor, its threshold value, and the fatigue strength of parts containing defects, including AM parts [130], [131], [132], [133]. This area is an effective area defined as a smooth contour circumscribing the irregular defect shape found on the fracture surface for various defect types based on fracture mechanics concepts,
Modeling the effect of defects on fatigue performance and life predictions of AM metals
Designing and building metal AM parts with required fatigue performance necessitates developing an integrated approach to link the process, structure, property, and performance [18]. Currently, modeling of these aspects, as well as topology and process optimization are main areas of research for AM [173]. Defect-sensitive fatigue life modeling is of great importance in linking AM metals fatigue performance to their structure and, subsequently, performance at the component level [174].
Numerous
Summary and perspective for future research
AM technologies have the potential of transforming the current manufacturing methods in the near future. The ongoing research could greatly facilitate the optimization of material design and AM process, based on industrial standards and qualification. Meanwhile, the cyclic mechanical properties of metal AM parts need to be evaluated and optimized since fatigue is a dominant mode of failure due to the intrinsic defects, as well as the cyclic nature of the loads applied to such parts. This was
CRediT authorship contribution statement
Niloofar Sanaei: Data curation, Formal analysis, Investigation, Visualization, Writing - original draft, Writing - review & editing. Ali Fatemi: Funding acquisition, Project administration, Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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